The isomerization of normal paraffins of low molecular weight is of high interest in the oil industry, in view of the particularly high octane number of the isoparaffins.
The n-C.sub.4 to C.sub.7 and mainly C.sub.5 -C.sub.6 paraffins can be advantageously converted to isoparaffins in order to obtain a motor-fuel of high octane number. By this process, the light gasoline fractions, in particular the straight-run top fractions, can be improved.
Three different types of isomerization processes exist:
the low temperature processes (operating at about 20.degree.-130.degree. C.) using a catalyst of the Friedel and Crafts type, such s aluminum chloride,
the "mean" temperature processes (operating at about 150.degree. C.) using as catalyst a supported metal such as platinum on halogenated alumina,
the high temperature processes (operating at 250.degree. C. or more), using zeolite carriers associated with a hydrogenating metal from group VIII.
The thermodynamic equilibrium is more favorable to isoparaffins at low temperature but the catalysts of the Friedel and Crafts type with halogenated alumina carrier, in view of their corrosive nature, require the use of reactors made of very costly special alloys.
The "high temperature" processes are the object, since about 20 years, of many patents. Most of the catalysts disclosed in these patents consist of zeolite, more particularly mordenite, in acid form, with or without hydrogenation promoters. SHELL Company was particulary interested in this process and has claimed catalysts of mordenite base, more particularly:
The mode of exchange of Na.sup.+ ions with NH.sub.4.sup.+ or H.sup.30 ions (U.S. Pat. No. 3,190,939) PA0 the thermal treatments with controlled moistness (U.S. Pat. No. 3,836,597--U.S. Pat. No. 3,842,114--U.S. Pat. No. 2,181,928). PA0 severe acid treatments: HCl 12N, 100.degree. C. (U.S. Pat. No. 3,480,539) PA0 self steaming (roasting in confined atmosphere) between 430.degree. C. and 820.degree. C., followed with acid etching (U.S. Pat. No. 3,506,400). PA0 depositon of metal on the modified zeolite, as in most of the above-mentioned patents. PA0 deposition of the one or more metals on an inert binder, for example alumina, and physical admixture with zeolite in protonic form (U.S. Pat. No. 3,432,568 to MOBIL, U.S. Pat. No. 3,632,835 to UOP, U.S. Pat. No. 4,374,296 to MOBIL). PA0 the total Si/Al atomic ratios are determined by X fluorescence, the sodium contents by atomic absorption. PA0 the mesh volume and the crystallinity are determined by X diffraction, the sample being prepared in a similar manner as in the operating mode of Standard ASTM D 3942 80 established for faujasite. PA0 the benzene adsorption capacity of the zeolite is determined by gravimetry. The sample is previously desorbed at 300.degree. C. under 10.sup.-4 Torr (133.32.times.10.sup.-4 Pa). PA0 as tetrammine complex by cationic exchange: the metal will then be deposited preferentially on mordenite, PA0 as hexachloroplatinic acid: by anionic exchange, the metal will be then preferentially deposited on alumina when alumina is the binder used during the shaping operation.
______________________________________ the acid treatment succession of hot acid, NH.sub.4.sup.+ (USP 3,442,794) succession of hot acid-cold acid (USP 3,475,345) acid solution containing Na.sup.+ or K.sup.+ ions (USP 2,272,737 - USP 4,359,409 - USP 4,400,576) ______________________________________
ESSO Company, on the other hand, has claimed the following mordenite dealumination processes:
The self steaming-acid etching succession for dealuminating zeolite is also found in U.S. Pat. No. 3,551,353 to MOBIL.
Two modes of metal deposition are considered:
Particular zeolite treatments, such as fluor introduction, have also been patented (U.S. Pat. No. 3,932,554 to NIPPON oil, U.S. Pat. No. 3,413,370 to UOP).
Mordenite is characterized by a Si/Al atomic ratio ranging generally from 4 to 6: its crystalline structure is formed of SiO.sub.4 and AlO.sub.4 basic tetrahedron chains, generating two types of channels: channels of dodecagonal opening (contour with 12 oxygens) and channels with octagonal opening (contour with 8 oxygens).
Two types of mordenite exist which differ from each other by their adsorption properties: the variety with wide pores, always synthetic, adsorbs such molecules as benzene (kinetic diameter=0.66 nm (0.66 nanometers) and the variety with small pores, natural or synthetic, which only adsorbs molecules of a kinetic diameter lower than about 0.44 nm. These mordenites also differ by morphological differences-needles for mordenite of small pores, spherulites for mordenite of wide pores--and by structural differences: presence or absence of defects. In the above-mentioned literature, the mordenite used is that of the wide pores type.
Now, the present invention has for object to use a mordenite of the wide pores type prepared from a mordenite of small pores in such operating conditions that the mordenite with wide pores, as used, will have kept the morphology of the mordenite with small pores while nevertheless having the capacity to adsorb the benzene molecule (kinetic diameter: 0.66 nm) at the difference of a mordenite with small pores, as above-mentioned. The use of said mordenite of particular morphology (needles), specially treated, gives a substantial increase in activity and selectivity for the isomerization reaction.
It is possible to "unclog" the channels of said particular zeolite by treatment with a strong inorganic acid and/or by roasting in the presence of steam and to obtain an adsorption capacity close to that of the mordenite of wide pores type.
These small pores synthetic mordenites may be obtained by synthesis, particularly in the following conditions: temperature from about 200.degree. to 300.degree. C. and crystallization time from 5 to 50 hours.